For high-end industrial applications, including more particularly as adhesives, pressure-sensitive adhesives or heat-sealing compositions, the ingredients used include polyacrylates, these polymers having emerged as being highly suitable for the growing requirements in these fields of application.
Thus adhesive compounds are required to have a good tack, but must also meet exacting requirements in the area of shear strength. At the same time, the processing properties must also be good, including in particular a high suitability for the coating of these compositions onto backing materials. This is achieved in particular by polyacrylates with a high molecular weight, high polarity and subsequent efficient crosslinking. Moreover, polyacrylates can be prepared transparently and with weathering stability.
In the coating of polyacrylate compositions from solution or as a dispersion, which can be used, for example, as a pressure-sensitive adhesive, viscoelastic backing or heat-sealing compositions, thermal crosslinking is well-established prior art. In general the thermal crosslinker—for example, a polyfunctional isocyanate, a metal chelate or a polyfunctional epoxide—is added to the solution of a polyacrylate furnished accordingly with functional groups, and this composition is coated in a planar fashion onto a substrate, with the aid of a doctor blade or coating bar, and is subsequently dried. As a result of this process, diluents—that is, organic solvents or water in the case of the dispersions—are evaporated and the polyacrylate, accordingly, is crosslinked. The crosslinking is very important for the coatings, since it gives them sufficient cohesion and thermal shear strength. In the absence of crosslinking, the coatings would be too soft and would flow away under even a low load. Critical to a good coating outcome is the observance of the pot life (processing life, within which the system is in a processable state), which can vary greatly according to crosslinking system. If this life is too short, the crosslinker has already undergone reaction in the polyacrylate solution; the solution is already incipiently crosslinked (partially gelled or completely gelled) and can no longer be coated out uniformly.
For reasons in particular of environmental protection, the technological operation for the preparation of pressure-sensitive adhesives is in a state of continual development. As a result of the environmental strictures, which have become more restrictive, and as a result of the climbing prices for solvents, there is concern as far as possible to eliminate the solvents from the manufacturing operation for polymers. In the industry, therefore, there is growing importance attached to melt processes (also referred to as hotmelt processes) with solvent-free coating technology for the preparation of polymers, particularly of pressure-sensitive adhesives. In such processes, meltable polymer compositions, in other words polymer compositions which at elevated temperatures undergo a transition to the fluid state without decomposing, are processed. Compositions of this kind can be processed outstandingly out of this melt state. In developments of this operation, the preparation as well can be carried out in a low-solvent or solvent-free procedure.
The introduction of the hotmelt technology is imposing increasing requirements on the adhesives. The aforementioned meltable polyacrylate compositions (other names: “polyacrylate hotmelts”, “acrylate hotmelts”) in particular are being very intensively investigated for improvements. In the coating of polyacrylate compositions from the melt, thermal crosslinking has to date not been very widespread, despite the potential advantages of this process.
To date acrylate hotmelts have primarily been crosslinked by radiation-chemical methods (UV irradiation, EBC irradiation). Yet this is a procedure fraught with disadvantages:                in the case of crosslinking by means of UV rays, only UV-transparent (UV-pervious) layers can be crosslinked;        in the case of crosslinking with electron beams (electron beam crosslinking or electron beam curing, also EBC), the electron beams possess only a limited depth of penetration, which is dependent on the density of the irradiated material and on the accelerator voltage;        in both of the aforementioned processes, the layers after crosslinking have a crosslinking profile, and the pressure-sensitive adhesive layer does not crosslink homogeneously.        
The pressure-sensitive adhesive layer must be relatively thin in order for well-crosslinked layers to be obtained. The thickness through which radiation can pass, though indeed varying as a function of density, accelerator voltage (EBC) and active wavelength (UV), is always highly limited; accordingly, it is not possible to effect crosslinking through layers of arbitrary thickness, and certainly not homogeneously.
Also known in the prior art are a number of processes for the thermal crosslinking of acrylate hotmelts. In each of these processes a crosslinker is added to the acrylate melt prior to coating, and then the composition is shaped and wound to form a roll.
Direct thermal crosslinking of acrylate hotmelt compositions containing NCO-reactive groups is described in EP 0 752 435 A1. The isocyanates used, which are free from blocking agents, and are, more particularly, sterically hindered and dimerized isocyanates, require very drastic crosslinking conditions, and so a rational technical reaction presents problems. Under the kind of conditions which prevail on processing from the melt, the procedure described in EP 0 752 435 A1 leads to rapid and relatively extensive crosslinking, and so processing of the composition, particularly with a view to the coating of backing materials, is difficult. In particular it is not possible to obtain any very homogeneous layers of adhesive of the kind that are needed for many technical applications of adhesive tapes.
Also prior art is the use of blocked isocyanates. A disadvantage of this approach is the release of blocking groups or fragments, which have an adverse effect on the adhesive properties. One example is U.S. Pat. No. 4,524,104 A. It describes pressure-sensitive acrylate hotmelt adhesives which can be crosslinked with blocked polyisocyanates together with cycloamidines or salts thereof as catalyst. In this system, the necessary catalyst, but especially the resultant HCN, phenol, caprolactam or the like, may significantly adversely affect the product properties. With this approach, moreover, there is a need for often drastic conditions for the release of the reactive groups. Significant product use is unknown to date and, furthermore, appears unattractive.
DE 10 2004 044 086 A1 describes a process for thermal crosslinking of acrylate hotmelts wherein a solvent-free functionalized acrylate copolymer which, following addition of a thermally reactive crosslinker, has a processing life which is sufficiently long for compounding, conveying and coating, is coated, preferably by means of a roller method, onto a web-like layer of a further material, more particularly a tapelike backing material or a layer of adhesive, and which, after coating, undergoes subsequent crosslinking under mild conditions until the cohesion achieved is sufficient for pressure-sensitive adhesive tapes.
A disadvantage of this process is that the reactivity of the crosslinker (isocyanate) predetermines the free processing life and the degree of crosslinking. Where isocyanates are used, they react in part during actual addition, as a result of which the gel-free time may be very short, depending on the system. A composition with a relatively high fraction of functional groups such as hydroxyl groups or carboxylic acid can in that case no longer be coated sufficiently well in the coatings. A streaky coat interspersed with gel particles, and therefore not homogeneous, would be the consesquence.
A further problem which arises is that the attainable degree of crosslinking is limited. If a higher degree of crosslinking is desired, through addition of a higher quantity of crosslinker, this has drawbacks when polyfunctional isocyanates are used. The composition would react too quickly and would be coatable, if at all, only with a very short processing life and hence at very high coating speeds, which would increase the problems of the non-homogeneous coating appearance.
DE 100 08 841 A1 describes polyacrylates which are obtainable through thermal crosslinking of a polymer mixture which comprises tert-butoxycarbonyl (BOC) protecting groups, a cationic photoinitiator and a difunctional isocyanate and/or difunctional epoxide. Also described is a process for preparing crosslinked polyacrylates, in which the polymers to be crosslinked are first protected by introduction of tert-butoxycarbonyl groups and the crosslinking takes place only after deprotection by thermal treatment of the polyacrylates that have then been deprotected. The introduction of the protecting groups in this case is to prevent the crosslinking reaction, which is only desired subsequently, when the operating temperatures prevailing are already high in the course of earlier stages of processing, as is the case, for example, in the hotmelt process. The protection is valid in particular for the crosslinking reaction at this point in time, but also for all other competing reactions which would attack the unprotected functional groups of the polymer to be processed, more particularly its hydroxide groups.
A disadvantage of the process presented therein is that the reactive groups, after coating, must first be released by UV irradiation. Consequently the disadvantages which apply here for thermal crosslinking are the same as those already recited above for radiation-induced crosslinking (UV irradiation). Moreover, combustible isobutene is released.
EP 1 317 499 A describes a process for crosslinking of polyacrylates via UV-initiated epoxide crosslinking, in which the polyacrylates have been functionalized during the polymerization with corresponding groups. The process offers advantages in relation to the shear strength of the crosslinked polyacrylates as compared with conventional crosslinking mechanisms, particularly as compared with electron beam crosslinking. This specification describes the use of difunctional or polyfunctional, oxygen-containing compounds, more particularly of difunctional or polyfunctional epoxides or alcohols, as crosslinking reagents for functionalized polyacrylates, more particularly functionalized pressure-sensitive acrylate hotmelt adhesives.
Since the crosslinking is initiated by UV rays, the resultant disadvantages are the same as those already mentioned.
Polyacrylate compositions, in particular pressure-sensitive polyacrylate hotmelt adhesives, have thus to date not been readily crosslinkable through crosslinking with polyfunctional epoxides, and this type of crosslinking can therefore not be used industrially for a production process.
It is an object of the invention to enable thermal crosslinking of polyacrylate compositions which can be processed from the melt (“polyacrylate hotmelts”), with a sufficiently long processing life (“pot life”) being available for the processing from the melt, especially as compared with the pot life of the known thermal crosslinking systems for polyacrylate hotmelts. At the same time, it ought to be possible not to use protecting groups which would have to be removed again, possibly, by actinic radiation or other methods. Moreover, it ought to be possible to set the degree of crosslinking of the polyacrylate composition to a desired level, without adversely affecting the advantages of the operating regime.
In the text below, the polyacrylate compounds are also referred to, synonymously and in short, as “polyacrylates”. For the non-crosslinked polyacrylate compositions, the term “addition polymers” is also used, with the term “polymers” being used for the crosslinked or incipiently crosslinked polyacrylate compositions.